i use this big kite for light winds and its the bigest one in the beach ,in 10-12 knots i can surf up wind with abig bord without working hard on the kite ,my problem is when i try jumping 'the jumps are very long but without hight (i use 20 m lines) ??

that's normal with a big kite. they are too slow to boost you up in the sky, therefore they can carry you longer through the air due to the large surface area. it is like a parachute. if you have a smaller kite, it turns faster and pulls you up more due to its speed. the highest jumps I did were with an ARX 7.5. My 15.5 has a longer airtime once up flying. But the 7.5 kicks me up and brings me down fast since the surface area has only 7.5 mÃ‚Â² which is not enough for my weight to make me fly longer.
So don't worry, it is not you!

don't gain too much pressure over the board and dont fly the kite too low (about 11 o' clock).
then steer it back as hard as you can, and when it passes 12 o' clock, kick off the water very hard and steer back as hard as you can ...

fly very agressive with your kite, but don't edge too hard, this should give you some extra inches of height in light winds ...

shorter lines make the kite faster.
I use for my 15.5 ARX 20 m lines and the kites flies great with it! But with bigger sizes the problem is, that the shorter the lines and bigger the kite, you will loose power and the kite can collapse, since the tips will be pushed together. The kite doesn't have its designed diameter anymore, therefore the stability will suffer.
You can try shorter lines and find out until which length it works.